Somebody who goes around all the women in the office and chats them up, regardless of whether or not they are single or want his attention. When they reject him, he will continue to pursue resulting in either success or a sexual harassment case.

Manager: "Why is John not at his desk?"
Secretary: "He's in a 'meeting' with Tessa."
Manager: "Ok, that makes sense. That John's a real sex pest. You'd better call him on the intercom."
Window cleaner: "Hey, that sex pest John has got a different woman spread out on the desk today"